The present invention relates to differential amplifier circuits, and in particular, to fully differential amplifier circuits with common mode feedback loops.
In many of today's complex electronic systems, reduction of noise is critical to achieving good circuit performance. One common technique is to use fully differential operational amplifiers to reduce sensitivity to power supply noise. Since both the input and output connections and signals are differential, a circuit is generally required to set the common mode of the output signal independently of the common mode of the input signal. This is often achieved by adding a common mode feedback loop that senses the output common mode and compares it to a desired value. The resulting common mode feedback error signal CMFB is fed back to the operational amplifier. Such a common mode loop also results in improvement in the power supply rejection ratio (PSRR). In most cases, the power supply noise is common mode in nature and, therefore, can be corrected by such a common mode loop configuration. Ideally, the differential and common mode signal paths will have similar open loop transfer functions thereby allowing the common mode loop to cancel noise introduced via the power supply connections.
However, in principal, this is often not true for at least two reasons. First, such a common mode loop usually requires an additional transfer function pole and needs to be compensated more heavily, thereby reducing its bandwidth. Second, when a small common mode signal is applied somewhere inside the loop, it gets amplified by the operational amplifier and the now gained up signal appears at the gate electrodes of the pair of differential amplifier transistors. Since the loop is common mode, these transistors do not amplify the signal, but simply pass it on to the drain of the tail current source. This results in modulation of the voltage at the drain electrode of the tail current source, thereby causing a common mode signal of a polarity opposite to the applied disturbance to become injected into the first stage of the loop. This, in turn, causes a reduction in loop gain at lower frequencies (which is dominated by the channel conductance of the current source), plus an additional phase shift added to the loop transfer function thereby causing further problems in stabilizing the loop.